Assembling yttrium-based high-temperature superconducting tapes in order to fabricate a large-scale magnet conductor has led to the National Institute for Fusion Science (NIFS) in Japan achieving an electrical current of 100,000 amperes, by far the highest in the world.

The researchers are working on a high-temperature superconducting coil appropriate for their fusion reactor magnet and they are doing it by stacking the tapes. For the conductor joints, which are important for the production of the large-scale coils, NIFS developed low-resistance joint technology through collaborative research with Tohoku University. As a result of the prototype conductor test, at the absolute temperature of 20 degrees Kelvin (minus 253 degrees Celsius) the electrical current exceeds 100,000 amperes.

The overall current density exceeds 40 A/mm2 including the jackets, and this value is of practical use for manufacturing large-scale fusion reactor magnets. This result is of global importance. We use 54 yttrium-based high-temperature superconducting tapes. Each tape is 10 mm in width and 0.2 mm in thickness. The cross-section of the superconductor sample is shown in Fig.1. The electrical current flows only through this area.

Together with the substrate used for this type of tape that is exceptional in strength and flexibility, by surrounding this area by a copper jacket and a stainless steel jacket an extremely strong conductor is produced. The whole set of the sample is shown in Fig.2. The current is induced by magnetic induction.

The method by which the helical fusion reactor's massive magnet is manufactured by sequentially connecting the short high-temperature superconductors has received much attention. The large current-capacity high-temperature superconductor with simple stacking of yttrium-based tapes and the so-called "joint winding method" have also impacted the development of high-temperature superconducting magnets used in medical instruments and power-electric devices.